The criminal justice system is failing to deliver value for money, spending watchdogs have warned, with victims facing delays, collapsed cases and a postcode lottery.

Two-thirds of cases do not progress as planned and there is "significant regional variation", the National Audit Office (NAO) said.

A victim of crime in North Wales has a seven in 10 chance that a trial will go ahead at crown court on the scheduled day - compared to two in 10 in Greater Manchester.

The NAO estimated that in 2014/15 the Crown Prosecution Service spent £21.5 million on preparing cases that were not heard in court.

Some £5.5 million of this sum related to cases that collapsed because of "prosecution reasons" - such as non-attendance by prosecution witnesses and incomplete case files.

Backlogs in the crown court increased by a third (34%) between March 2013 and September last year and the waiting time for a hearing jumped by 35% from 99 days to 134 over the same period.

Amyas Morse, head of the NAO, said: "Delays and aborted hearings create extra work, waste scarce resources, and undermine confidence in the system.

"Some of the challenges are longstanding and complex - others are the results of basic avoidable mistakes."

Delays are "getting worse against a backdrop of continuing financial pressure", with central government spending on the system at around £2 billion a year - a fall of 26% in real terms since 2010/11, according to the report.

It concluded that the system as a whole is "inefficient" because its individual parts have "strong incentives" to work in ways that create costs elsewhere.

For instance, courts staff acting under judicial direction seek to ensure that courts are in use as much as possible by scheduling more trials than can be heard so that there are back ups when one trial cannot proceed.

The NAO said delays can increase costs in other areas.

For example, London police officers who spend a day waiting to give evidence cost £139 a day.

If an officer attends every cases that "cracks" - meaning a trial is withdrawn on the day it is due to start and is not re-listed - this could amount to £10.6 million in wasted police time.

The report also highlighted the potential for delays and collapsed trials to undermine confidence in the system, with surveys suggesting that only 55% of people who have been a witness or victim would be prepared to act as a witness again.

The NAO said that although the volume of cases entering the system is reducing, they are becoming more complex and resource-intensive.

In the last five years there has been a 12% rise in sex offence cases in the crown court, including historic abuse and child sex allegations involving vulnerable victims and witnesses.

Prosecutions for other serious offences are also increasing, such as terrorism, organised crime, drugs and fraud. The average length of a crown court trial increased from 11.5 hours in 2010/11 to 14.6 hours in the year to September.

There was some improvement in the management of cases since 2010/11, with the proportion of effective trials in the magistrate's court increasing.

The NAO concluded that "ambitious" reforms led by the Ministry of Justice, CPS and judiciary have the potential to improve value for money but warned they "will not address all of the causes of inefficiency".

A CPS spokeswoman said: "Providing swifter and more effective justice is a priority for the Crown Prosecution Service and we are working closely with our criminal justice partners to deliver this.

"Significant work has been undertaken to improve the efficiency of the CPS, and while our conviction rate has remained steady at over 80% in the face of wider challenges, there is still scope for more improvement.

"Our ambitious programme of reform is already under way, and these changes are expected to make a real difference to the efficiency of cases."

A Ministry of Justice spokeswoman said: "We welcome this report and will reflect on its recommendations.

"The MoJ is embarking on a radical reform of the criminal justice system. As the Justice Secretary has said, our criminal justice system is in need of urgent reform.

"Our courts are archaic and slow, and their out of date processes do not meet the needs of the public.

"That is why we are investing a record £700 million to build a justice system that is swifter and more certain.

"We will use modern technology to meet the needs of everyone who uses our services."

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